Secondary batteries employing sulfur electrodes operate at relatively high temperatures which accelerate the corrosion of cell materials which contact the sulfur, e.g., the container/current collector. Container corrosion can have diverse effects: apart from attacking the material and "eating up" active reactant (thereby reducing capacity), it forms surface layers that increase contact resistance to the electrode normally used in such batteries. Also soluble corrosion products can be redeposited, clogging the electrode, obstructing transport of active materials, and causing uneven current distribution. If deposited on the electrode surface, they change its structure and wettability and therewith the kinetics of reactions occurring at this surface to the point where it may become partially or completely blocked. Corrosion products can also be deposited on the surface of the solid electrolyte partially blocking, damaging or destroying it by causing locally excessive current densities or by direct interaction or penetration.
In an attempt to overcome these corrosion problems, a variety of electrically conductive materials have been proposed as coatings for the container current collector in a Na/S battery.
In U.S. Pat. No. 3,959,013, proposal is made to use a corrosion resistant and electronically conducting layer of molybdenum or graphite to reduce the corrosion associated with such a battery. The corrosion resistant coating is placed on the surface of the metallic container which is to confine the molten sodium polysulfide and sulfur reactants.
In a similar manner, U.S. Pat. No. 4,048,390 suggests that a protective coating of aluminum be placed on the surface of the battery container which is to confine the polysulfide and sulfur reactants. This patent proposes the use of aluminum because it forms a continuous layer of aluminum sulfide over its exposed surfaces.
U.S. Pat. No. 4,110,516 takes still another direction in trying to develop a corrosion resistant container to confine the sodium polysulfide and sulfur reactants. The patent suggests forming the confining container of aluminum and then placing over the aluminum either a single layer of chrome or a layer of zinc and a layer of chrome thereover, the chrome surface being the surface which faces up against the corrosive reactants which are to be confined therewithin.
U.S. Pat. No. 4,131,226 once again discloses a sulfur electrode container for a sodium sulfur battery in which a liner material is used as an anticorrosive surface for a mild steel container. The patent teaches that the discrete liner of clad material can be formed of metal such as stainless steel, molybdenum or a nickel/chromium alloy, as specifically disclosed therein.
In U.S. Pat. No. 4,160,069, the current collector comprises a corrosion resistant ceramic member and an intimately attached metal cladding. The ceramics employed comprise doped rutile TiO.sub.2, doped calcium titanate and lanthanum strontium chromite.
U.S. Pat. No. 4,216,275 attempts to overcome the corrosive nature of the polysulfide melt of a Na/S battery by providing a light metal cell wall which is coated first with a prime coat of nickel and aluminum, and then applying on this prime coat, a coating of an alloy of chromium and at least one metal of the group iron, cobalt or nickel.
U.S. Pat. No. 4,226,922 suggests that longevity of the cathodic current collector can be obtained if the metallic current collector has a boronized surface and an additional boron source in physical proximity to the boronized current collector surface.
Still another approach to forming a non-corrosive, electrically conductive component for a sodium sulfur cell is taught in U.S. Pat. No 4,232,098. The component comprises a fiber-carbon substrate and a non-porous chromium-iron-carbon duplex alloy surface layer chemically diffusion bonded to the substrate.
In U.S. Pat. No. 4,248,943 a coating of chromium/chromium oxide is placed on the surfaces of the electrically conducting components of a Na/S battery to combat corrosion by molten sodium polysulfide and sulfur reactant.
Few conductive materials can withstand the attack of the polysulfide melt at the operating temperature of the Na/S cell.
Metals are thermodynamically unstable. They form sulfides, whose solubility in the melt is, in most cases, not negligible. Some metals like chromium, molybdenum tungsten, and aluminum become covered by protective layers which, however, lose their passivating properties under certain conditions. Ceramic materials such as chromium oxide, which exhibit good corrosion stability in polysulfide melts, do not however have sufficient electronic conductivity to be employed in current collectors. Dopants are sometimes employed in ceramics to increase the conductivity of the ceramics. However some dopants, e.g., NiO, while increasing conductivity of Cr.sub.2 O.sub.3, compromise its corrosion resistance.
Useful coatings employed on the container/current collector of a Na/S cell must have good corrosion stability to the polysulfide melt, sufficient conductivity, and preferential wettability for sodium polysulfide rather than sulfur. It has not been found that chromium oxide (Cr.sub.2 O.sub.3) material doped with magnesium oxide, MgO, satisfies these requirements. In addition to these inherent material properties, chronium oxide coatings display the following characteristics: adherence to the substrate under conditions of thermal cycling, cost viability and a relatively simple method of deposition on the substrate.